1. Field of the Invention
This invention relates to a device for detecting the phase of an input signal, and more particularly to a device suitable for use in a demodulator for QPSK (Quadrature Phase Shift Keying) modulated signals.
2. Description of the Related Art
The PSK (Phase Shift Keying) modulation system is a modulation system which discretely varies the phase of a carrier according to digital codes. This modulation system is widely used in satellite communication fields because of its low error rate and its narrow occupied bandwidth. In the PSK modulation system, a basic modulation system is a binary PSK modulation (BPSK) system. In the BPSK modulation system, phase data "0" and ".pi." are respectively allocated to binary data "1" and "0". That is, in the BPSK modulation system, a carrier is transmitted without any phase shift when datum to be transmitted is "1" while a carrier is transmitted with a phase shift of .pi. when datum to be transmitted is "0".
The 4-phase PSK (QPSK) modulation system is widely used as a PSK modulation system. In the QPSK modulation system, four phase shift data "0", ".pi./2", ".pi.", and "3 .pi./2" are for example allocated to four combinations of two bits. That is, in the QPSK modulation system, a carrier is transmitted without any phase shift when two bits to be transmitted is (00). A carrier is transmitted without with a phase shift of .pi./2 when two bits to be transmitted is (01). A carrier is transmitted without with a phase shift of .pi. when two bits to be transmitted is (11). A carrier is transmitted without with a phase shift of 3.pi./2 when two bits to be transmitted is (10). Signal points corresponding to 2-bit data to be transmitted is shown in FIG. 6.
An example of the configuration of a modulator which modulates the QPSK signal is shown in FIG. 7.
Referring to FIG. 7, the mixer (MIX) 204 mixes a QPSK signal received as an input signal with a local oscillation signal supplied from the local oscillator (OSC) 202. The local oscillation signal is a signal in synchronism with the input signal S. The mixer 204 outputs the sum of two input signals and the difference of two input signals. The low-pass filter (LPF) 206 extracts only the difference signal component and inputs it as the I component of the input signal S to the analog to digital (A/D) converter 210.
The .pi./2 phase shifter 203 shifts a local oscillation signal from the local oscillator (OSC) by .pi./2. The mixer 205 receives the input signal S and the local oscillation signal shifted by .pi./2. Hence, the mixer 205 outputs the sum and difference frequency signals of the input signal S and the .pi./2 phase shifted local oscillation signal. The low-pass filter (LPF) 207 extracts only the difference signal component and outputs it as the Q component of the input signal S to the analog to digital (A/D) converter 211.
The I component 212 from the A/D converter 210 and the Q component 213 from the A/D converter 211 are supplied as address signals to the ROM look-up table 214. The I component 212 is for example a 8-bit digital signal while the Q component 213 is for example a 8-bit digital signal. The ROM look-up table 214 stores phase shift angles. The ROM look-up table 214 receives as an address the I component 212 and the Q component 213 and outputs the phase angle .theta. of the input signal S. The phase decision section 216 receives the output 215 from the table 214 and then outputs 2-bit data (X, Y) corresponding to the phase angle .theta.. Thus, the input signal S QPSK modulated is demodulated.
When the input signal S is input as shown in FIG. 6, the phase decision section 216 receives data on the phase angle .theta. read out of the ROM lookup table 214 and outputs demodulation data (0, 0). The ROM look-up table 214 stores data on phase angles 0 to 2.pi.. Since 2-bit data can be transmitted as one symbol in the QPSK modulation system, high rate transmission can be performed with a narrow transmission frequency band without an increase in the modulation rate.
Where the address used to refer to the ROM look-up table 214 is represented in 8 bits and the phase angle data read out of the ROM look-up table is represented in 8 bits, it is needed that the ROM forming the ROM look-up table 214 has a memory capacity of 4 k words. The ROM forming the ROM look-up table 214 is generally built on the demodulation IC chip.
Hence, there is the problem in that the use of a 64 k-word ROM results in increasing the implementing area as well as the unit price of the demodulation IC chip.